What was once a series has become a war. Every basket for a purpose now. Every rebound for a cause. The innocence of these play-off games has come crashing down like a piano from a 10th- floor window, and you need only visit any sports bar in this country today to know what we are talking about.

Tuesday night. Tuesday night. It was Tuesday night that blew the lid off this Pistons-Celtics box of warfare, Tuesday night that sucked in the whole nation. Wasn’t it? Don’t you figure? Tuesday night? Game 5?

Oh sure, things were bubbling even earlier: Larry Bird and Bill Laimbeer, clawing and jawing in Game 3; Robert Parish predicting the Pistons “will choke” after Game 4; Isiah Thomas glowering in the Silverdome, saying, “This is our house; they don’t win here.”

All that was swirling in the pot, growing hotter and hotter, and then . . . Tuesday night. The Pistons do the impossible, they have the Celtics beaten at Boston Garden; and in five maddening seconds it all slips away. Is there anyone in either city who hasn’t seen “The Play” at least 100 times by now? Larry Bird steals Thomas’ inbounds pass. Larry Bird dishes to Dennis Johnson. Dennis Johnson lays it in and blows up Boston Garden.

“Lemme tell you something,” someone begins, “the Pistons know they had them beat. No way they let their season end like that. They could have won all five games in this thing. They’re gonna come back stronger than ever.”

“Lemme tell you something,” comes the answer. “You don’t recover from bullets. You don’t recover from bombshells. The Celtics have got it. You can forget it.”

DETROIT-BOSTON.

Game 6.

Look out.

How tense has this series become? Players are paying to stay in it. Laimbeer was fined $5,000 for grabbing Bird, and Bird was fined $2,000 for throwing punches, and Parish was fined $7,500 and suspended tonight for making like Muhammad Ali on Tuesday. Fighting? Fines? This is basketball?

More than basketball. Tradition. Pride. Town against town. “I’m sure all of Detroit thought we had Game 5 won, and then — whammo!” Chuck Daly said Wednesday. Whammo. And the whole town gears up for the next round.

Isn’t this already better than you ever figured? Even before they tap it up tonight? Here is a city that was wiping its brow from the Red Wings’ hockey miracles and — pow! Basketball. Big after big. Sinatra following Elvis. Or Elvis following Sinatra. Whoever. And the same spirit that watched the Edmonton Oilers and said, “We can beat these guys!” has resurfaced in a domed stadium 40 miles up the highway.

“What do you do now?” someone had asked Isiah Thomas after Tuesday’s loss, as he slumped by his locker.

“What we do now,” he had said, softly, deliberately, “is go back home and win, then come back here for Game 7.”

Is there anything else? Isn’t that what it is all about? The finish line? Game 7? The Pistons, for the first time, are staring down the barrel of their

season’s end. They are not ready for it yet. Every basket tonight will be an attempt to forget the nightmare Johnson laid in on Tuesday; every rebound will be a fight to go back to where the nightmare happened. Back to Boston, back to the haunted house, back to the Garden.

ONE MORE game.

There could be no better resolution for this series. It is a growing thing now, it has taken on life, it draws blood, it starts arguments. It is Laimbeer’s scuffles, Thomas’ persistence, Bird’s jumper, Parish’s injured ankle. It is Adrian Dantley with that look in his eye. Rick Mahorn on Kevin McHale. Coaches Chuck Daly and K.C. Jones crossing their arms and staring at the referees in disbelief.

It is passion against pride. Resistance against persistence. A team that owns it against a team that wants it. Every basket with a purpose now. Every rebound a cause.

“Lemme tell you something,” comes a voice. “Tonight is gonna be a whale of a game. All the anger. All the boasting. All the fights. Everything they got. Both of them.”

“Lemme tell you something . . . ” comes the other voice, “I agree.”

Detroit-Boston.

Game 6.

Whoo boy.

CUTLINE Bill Laimbeer goes down as he is hit by Boston’s Robert Parish during the second quarter of action at Boston Garden on Tuesday night. The Celtics’ Robert Parish is suspended from tonight’s game. Full coverage, 1D. The poster above was hanging at a downtown store.